In recent years, with higher integration of semiconductor devices, patterns finer than the transfer limit of the photolithography technique using the conventional ultraviolet light have been required in the semiconductor industry. In order to enable transfer of such fine patterns, the EUV lithography being an exposure technique using extreme ultraviolet light (hereinafter referred to as EUV light) with a shorter wavelength is expected to be promising. Herein, the EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet region and, specifically, light having a wavelength of about 0.2 nm to 100 nm.
As an exposure mask for use in this EUV lithography, use is normally made of a reflective mask in which a multilayer reflective film (multilayer film) for reflecting the EUV light serving as exposure light is formed on a substrate (base substrate) and, further, an absorber film (metal film) for absorbing the EUV light is formed in a pattern on the multilayer reflective film.
When pattern transfer is carried out using an exposure apparatus (pattern transfer apparatus) with such a reflective mask mounted therein, the exposure light incident on the reflective mask is absorbed at a portion where the absorber film pattern is present, while, is reflected by the multilayer reflective film at a portion where the absorber film pattern is not present so that the reflected light is transferred to, for example, a semiconductor substrate (resist-coated silicon wafer) through a reflective optical system.
On the other hand, with the increasing demand for miniaturization in the photolithography process, problems in the photolithography process used in the semiconductor manufacturing processes are becoming remarkable. As one of them, there is cited a problem of a defect of a photomask substrate formed with a pattern, which is used in the photolithography process.
Conventionally, the existing position of a defect of a photomask substrate is specified by the distance from an end face of the substrate. As a consequence, the positional accuracy is low and thus, when patterning a light-shielding film while avoiding the defect, it is difficult to avoid it on the order of μm. Therefore, the defect is avoided by changing the direction of pattern transfer or roughly shifting the pattern transfer position on the order of mm.
Under these circumstances, for the purpose of increasing the inspection accuracy of a defect position in a light-transmitting mask, there is an attempt to form a reference mark on a transparent substrate and to specify a position of a defect using the reference mark as a reference position (see, e.g. JP-A-2003-248299 (Patent Document 1)). In the invention described in this publication, the reference mark is formed to be very shallow (depth is about 3 μm) so as to prevent dust from staying in a recess serving as the reference mark. In the case of a normal light-transmitting mask, although a thin film such as a light-shielding film is formed on an upper surface of a reference mark, since it comprises a single layer or about several layers, even if the depth of the reference mark is shallow, it is possible to obtain contrast for inspection light (deep-ultraviolet light of about 190 to 260 nm, or the like) and thus to recognize a position of the reference mark in a defect inspection.